A CZ method is one of the methods for producing single crystal silicon.
With this method a single crystal silicon ingot is produced by providing quartz crucible in a container for pulling single crystals, that is, a CZ furnace, heating and melting polycrystalline silicon inside the quartz crucible, dipping a seed crystal mounted on a seed chuck into the melt once the melt has stabilized and growing single crystal silicon by pulling up the seed chuck, while rotating the seed chuck and the crucible in the same direction or opposite directions.
“Slip dislocations” is one of the unavoidable problems encountered when single crystal silicon is grown by the CZ method. Slip dislocations are one-dimensional crystal defects that appear due to thermal stresses occurring when the seed crystal is brought into contact with the melt and they propagate in a fixed direction.
If slip dislocations are included in grown single crystal silicon, quality of the semiconductor device fabricated based on the single crystal silicon is degraded. Therefore, such slip dislocations have to be eliminated.
A technology for eliminating slip dislocations has been established with respect to the production of silicon wafers (<100> axis crystal) with the {100} crystal plane as the surface. Thus, when the seed crystal is pulled up so that the <100> crystal orientation matches the axial direction of the seed crystal, slip dislocations can be easily eliminated from the single crystal silicon by applying necking process which gradually reduces the diameter of the single crystal silicon after the seed crystal has been brought into contact with the melt.
However, it was found that slip dislocations are difficult to eliminate in the production of silicon wafers (<110> axis crystal) with the {110} crystal plane as the surface, that is, when the seed crystal is pulled up so that the <110> crystal orientation matches the axial direction of the seed crystal, and the technology for eliminating slip dislocations in such a process has not yet been established.
When pulling is so conducted that the <110> crystal orientation matches the axial direction of the seed crystal, even if the diameter of the single crystal silicon is appropriately reduced by the necking process, dislocations easily remain in the central portion of the crystal, causing defects in semiconductor devices. Slip dislocations cannot be eliminated unless the diameter of the single crystal silicon is appropriately reduced to a level surpassing that attained when the <100> axis crystal is pulled up.
However, a demand for producing large-diameter silicon wafers has been created in recent years. Accordingly, single crystal silicon ingots with a large diameter and heavy weight have to be pulled up without problems, and if the diameter of the single crystal silicon is reduced, even though slip dislocations can be eliminated to a certain degree, there is a risk of the diameter being too small and making it impossible to pull large-diameter heavy single crystal silicon ingots.
There is a technology called a magnetic field applied Czochralski method (MCZ method). With this method, stable crystal growth is conducted by applying a magnetic field to a melt in order to increase melt viscosity and suppress convection in the melt.
When a magnetic field applied Czochralski method is employed and single crystal silicon is pulled, while a magnetic field is applied to the melt, slip dislocations cannot be eliminated unless the diameter of the single crystal silicon is further reduced by comparison with the process without magnetic field application. Thus, it was experimentally established that slip dislocations cannot be eliminated if the diameter of the single crystal silicon is not reduced to about 2.5 mm.
Japanese Patent Application Laid-open No. 9-165298 describes an invention according to which when pulling is so conducted that the <110> crystal orientation matches the axial direction of seed crystal, slip dislocations are eliminated by reducing the diameter to less than 2.0 mm with a necking process, while single crystal silicon is pulled with a magnetic field being applied by a magnetic field applied Czochralski method.
However, if the invention described in this publication is applied to pulling single crystal silicon ingots of a large diameter and a heavy weight, there is a risk of the necking portion breaking and the crystal falling down, which makes it impossible to use this process.
Further, U.S. Pat. No. 4,002,523 describes a technology for eliminating slip dislocations by obtaining a special shape of single crystal silicon rather than by simply reducing the diameter thereof. This patent discloses a technology for eliminating slip dislocations by conducting multistage cross-section area reduction in a necking process and obtaining a “bulge shape”.
However, though obtaining the bulge shape is technologically possible, this is actually difficult to do in an automated process.
With the foregoing in view, a technology is required which is suitable for eliminating slip dislocations by a method other than the necking process (cross-section area reduction) and for trouble-free pulling single crystal silicon ingots which have large diameter and heavy weight.
Further, technologies designed to eliminate defects including dislocations by methods other than the necking process (cross-section area reduction) have been disclosed in a variety of open publications.
Japanese Patent Application Laid-open No. 57-17494 describes an invention according to which when single crystals of compound semiconductors such as InSb are grown, etch pits are eliminated and uniformity of impurity concentration is improved by tilting the pull direction of the seed crystal at an angle of 5–10 degrees with respect to the <110> crystal orientation and pulling the single crystal of the same compound semiconductor.
However, though this publication describes the elimination of etch pits, it does not describe the elimination of slip dislocations. Further, the invention described in this open publication is developed to pull the single crystals of compound semiconductors such as InSb, and is not aimed at preparation of silicon single crystals.
Further, Japanese Patent Application Laid-open No. 3-80184 describes an invention according to which when a single crystal of a compound semiconductor such as GaAs is similarly grown, the single crystal of this compound semiconductor is pulled by setting the pull direction of the seed crystal to any direction between the <001> crystal orientation and <101> crystal orientation and axial dislocations propagating strictly in the growth direction are eliminated. Further, with respect to slip dislocations occurring in the compound semiconductors, this publication describes that “the generation of slip dislocation can be reduced by adding an impurity”.
Thus, though this publication describes the elimination of slip dislocations generated in a compound semiconductor such as GaAs by the addition of an impurity, it does not describe a technology for eliminating slip dislocations appearing in single crystal silicon.